The present invention relates to a radio communication system for radio communication which comprises a base station and a mobile terminal, and is based on a COMA system.
An IS-95A system and an IS-95B system are well-known as such a radio communication system. In the IS-95A system, a radio line which a mobile terminal uses for communication is called a traffic channel. In the CDMA method, necessary qualifies of communication through communication channels must be at the lowest level so as to assure a line capacity. For this purpose, a fast power control is requested for, in particular, a reverse line. In the IS-95A system, in order to assure the line capacity, a power control signal “power control bit” is inserted to a forward traffic channel to receive signals from all mobile terminals at a base station with the same quality, thereby controlling transmission powers of the mobile terminals.
If the signal received by one base station has an insufficient-quality signal, the base station inserts “0” to the forward traffic channel as the power control bit and transmits it. If the signal received by one base station has an excessive-quality signal, the base station inserts “1” to the forward traffic channel as the power control bit and transmits it. When the power control bit “0” is received, the mobile terminal increases the transmission power. When the power control bit “1” is received, the mobile terminal decreases the transmission power. In the IS-95A system, because a maximum value of the transmission power of the mobile terminal is determined, the transmission power of the mobile terminal does not exceeds the maximum value if a state of the power control bit “0” continues.
On the other hand, in the IS-95B system, in order to perform data communication faster, a plurality of traffic channels for data communication can be assigned to one mobile terminal. It is assumed that in the IS-95B system, an existing traffic channel used in the IS-95A system is called a fundamental channel and a traffic channel added for data communication is called a supplemental channel. One fundamental channel is necessarily assigned to the mobile terminal during communication. The maximum of seven supplemental channels can be assigned to the mobile terminal during communication.
A diffusion code for the fundamental channel is different from that for the supplemental channel. On the other hand, the power control bit is inserted only to a forward fundamental channel. When a reverse supplemental channel is assigned, a diffusion unit for the fundamental channel and a plurality of diffusion units for the supplemental channel are set to one mobile terminal. Thus, the mobile terminal uses a plurality of traffic channels, thereby enabling the data communication.
Referring to FIG. 1, the fundamental channels and the reverse supplemental channel are assigned to a mobile terminal PS. When a signal is transmitted from the mobile terminal PS, a base station BS measures a receiving level or line quality of data transmitted from the mobile terminal PS through the fundamental channel. If the receiving level or line quality measured are compared with a target value and the compared result is then sufficient, the transmission power of the mobile terminal PS is controlled to decrease the transmission power. If the compared result is insufficient, the transmission power of the mobile terminal PS is controlled by using the power control bit to increase the transmission power.
If the mobile terminal PS is located far from the base station and the propagation loss of the radio line increases, the base station BS cannot receive the signal from the mobile terminal PS at the sufficient level or quality. In this case, the base station BS controls the transmission power of the mobile terminal PS to be increased by use of the power control bit to set the transmitted power to the sufficient receiving-level or receiving quality. Further, when the mobile terminal PS moves and the propagation loss of the radio line increases, the base station BS also controls the transmission power of the mobile terminal PS to be increased.
However, the transmission power of the mobile terminal PS has the upper limit and the transmission power of the mobile terminal PS cannot be increased though the base station instructs the increase in transmission power, as shown in FIG. 2. As a result, the base station BS cannot receive the reverse signal from the mobile terminal PS. If such a state continues, a call of the mobile terminal PS is disconnected, the mobile terminal PS continues the transmission of the maximum transmission power to increase the interference with the reverse signal of other mobile terminals.